The present invention relates to the general field of controlling the orientation of fan blades of a turbine engine. A preferred field of application of the invention is that of two-propeller airplane turboprops.
A two-propeller airplane turboprop comprises a turbine having two contrarotating rotors, each driving a respective set of unducted fan blades. In that type of aeroengine the angle of orientation of the fan blades (also known as their pitch) constitutes one of the parameters used for managing the thrust of the engine.
French patent application number 10/51458 filed on Mar. 1, 2010 by the Applicant describes a system for controlling the orientation of the fan blades of such a turboprop. In that document, each blade is coupled, for the purpose of adjusting its orientation, to a blade root support that is pivotally mounted on a rotary ring by means of bevel gearing, one of the gears of that gearing being supported by the blade root support and the other gear and carrying a counterweight. A hydraulic actuator that is centered on the axis of rotation of the rotary ring and that is constrained to rotate together with the turbine rotor is connected to each of the counterweights via a respective radial link. By actuating the actuator, the links are caused to act on the counterweights on a rod/crank principal in order to cause the blade root supports to turn in synchronized manner. Such a control system presents numerous advantages. In particular, it is reliable and light in weight since it possesses relatively few parts compared with prior art control systems.
Changing the orientation of the blades between stages of flight and stages in which the airplane has its wheels on the ground (idling on the ground, taxiing, or indeed moving the blades into a thrust-reversal position) involves passing through a zone in which the pitch of the propellers is small and gives rise to a large amount of drag.
Thus, in order to avoid excessive drag, while also complying with the changeover time required for passing into the thrust-reversal position, it is necessary to double the speed of movement of the actuators actuating the pitch-changing mechanisms of the propellers of the rotors.
This increase in the speed of movement, combined with a high operating pressure, leads to additional mechanical power being taken off (in particular while idling) and to large amounts of heat being given off by the hydraulic pump that controls the actuators. This has a major impact on dimensioning the equipment in the oil circuit (heat exchangers . . . ), and the amount of power taken from the engines can be unacceptable.